Turn broaching machines have recently been used to a great extent for machining crankshafts, camshafts and similar parts. The workpiece, which is usually clamped and driven on both ends, rotates rapidly, and the blades which sit on the perimeter of a disk-shaped tool are engaged on the workpiece. The disk-shaped tool rotates relatively slowly about an axle parallel to the longitudinal axis of the workpiece. The individual blades on the periphery of the disk-shaped tool may all be the same distance from the point of rotation or they may be arranged with increasing distances. In the case where the blades are arranged on an arc and are all the same distance from the point of rotation of the disk-shaped tool, the tool must be advanced in the X direction, i.e., toward the axis of rotation of the workpiece. However, if the blades are arranged so the distance from the point of rotation of the tool increases, then the advance is determined only by the rotation of the tool.
Depending on the given machining application, the blades extend over the entire periphery of the disk-shaped tool or only over a part of the circumference.
This machining principle is used especially for final machining of the bearing faces of crankshafts and camshafts because the cutting pressure acting on the workpiece is much lower than in turning, so these relatively labile workpieces are also subjected to relatively little deformation during machining. In addition, this machining process yields very good surface quality.
Thus far two different types of machines are known for performing turn broaching.
The so-called set-type machines are suitable for holding turn broaching tool sets. These are several disk-shaped individual tools which are arranged together in parallel at a certain distance and are joined by an approximately cylindrical central body with a much smaller diameter so they are all driven together. The central body can also be divided in the longitudinal direction, i.e., a plane parallel to the Z direction, just like each individual tool. The two half-shells are placed on a specially designed carrying shaft when tooling the turn broaching machine, so the carrying shaft runs in the Z direction and joins the two tool supports together in one piece. These two tool supports cannot be lengthened in their mutual position in the Z direction in a so-called set machine and this form a unit with a U-shaped outline and a fixed spacing in the Z direction. Regardless of whether only such tools whose blades have an increasing distance from the center of rotation are used, the tool support can either be moved in the X direction or not. With tool supports that cannot be moved in the X direction, the construction cost of the machine is lower, but on the other hand a specially designed tool must be created for each new application. As a rule, since the main bearing points of such crankshafts are usually aligned with each other, when they must be produced over a long period of time and in large numbers they are therefore finished in one operation.
However, if a number of different workpieces which change frequently must be machined by the turn broaching principle, so-called flexible turn broaching machines are used. These are turn broaching machines that have at least two tool supports that can be moved independently of each other in both X and Z directions, so one disk-shaped tool can be driven in each tool support.
Due to the fact that the tool supports and thus the tools can be moved in both X and Z directions, any rotationally symmetrical surfaces on a wide variety of workpieces can be machined. However, since only two tools, for example, are in use at the same time, the progress made in machining is not as fast as when four or more disk-shaped tools of a turn broaching tool set are in use simultaneously.
Thus these flexible turn broaching machines are especially suitable for machining reciprocating bearings, for remachining individual main bearings or for machining recesses at the transition from the bearing face to the cheek.
In flexible turn broaching machines, the steep taper which must be provided at the end of the spindle with the facing stop attached to it is used as the tool holding fixture which must be aligned in the Z direction, or a standardized jaw chuck is used such as that also used for a spindle head chuck. The disk-shaped tool as well as the central body to which it is attached therefore need no longer be divided in a plane parallel to the Z direction but instead the central body need only be designed on the side facing the tool support so it can be secured and chucked on the tool holding fixture.
This eliminates a number of disadvantages such as those encountered with tool sets that can be divided longitudinally as on such set machines because the half-shells used there must be placed on a specially designed carrier shaft and anchored there, i.e., they must be chucked, so an extremely precise alignment of the two shells in both X and Z directions is required for satisfactory machining results. In addition, a very high gripping power must be applied to secure the shells on the carrying shaft. On the other hand, due to the one-sided chucking of a disk-shaped tool and its central body in a jaw chuck, there cannot be any bilateral chucking of the tool or central body with all the resulting disadvantages.